Uncertainity Principle

Uncertainity principle says that "If a measurement of position is made with accuracy Δx and if the measurement of momentum is made simultaneously with accuracy Δp , then the product of two errors can never be smaller than a number of order h
ΔpΔx≥(∼h)                                  (1)
Similarly if the energy of the syatem is measured to accuracy ΔE , then time to which this measurement refers must have a minimum uncertainity given by
ΔEΔt≥(∼h)                                    (2)
In generalised sence we can say that if  Δq is the error in the measurement of any co-ordinate and Δp is the error in its canonically conjugate momentum then we have,
ΔpΔq≥(∼h)                                  (3)
Consider the relation between the range of position Δx and range of wave number Δk appearing in a wave packet then
ΔxΔk≥1                                                     (4)
and this is a general property not restricted to quantum mechanics. Uncertainity principle is obtaines when the following quantum mechanical interpretation of quantities appearing in above equation are taken into account.
(1) The de-Brogli equation p=hk creates a relationship between wave number and momentum , which is not present in classical mechanics.
(2) Whenever either the momentum or the position of an electron is measured , the result is always some definite number. A classical wave packet always covers a range of positions and range of wave numbers.
Δx is a measure of minimum uncertainity or lack of complete determination of the position that can be ascribed to the electron. and Δk is the measure of minimum uncertainity or lack of complete determination of the momentum that can be ascribed to it.

Relation of spreading wave packet to uncertainity principle
Narrower the wave packet to begin with , the more rapidly it spreads. Because of the confinement of the packet within the region Δx0 the fourier analysis contains many waves of length of order of Δx0 , hence momenta p≅h/Δx0
therefore
Δv≅p/m≅h/mΔx0
Although average velocity of the packet is equal to the group velocity , there is still a strong chance that the actual velocity will fluctuate about this average by the same amount. The distance covered by the particle is not completely determined but it may vary as much as
Δx≅tΔv≅ht/mΔx0
The spread of the wave packet may therefore be regarded as one of the manifestations of the lack of complete determination of initial velocity necesarily associated with the narrow wave packet.

Relation of stability of atom to uncertainity principle
From uncertainity principle if an electron is localized it must have on an average a high momentum and have high kinetic energy as it takes energy to localize a particle. According to uncertainity principle it takes a momentum  Δp≅h/Δx and an energy nearly equal to h2/2m(Δx)2 to keep an electron localised within a region Δx. Momentum creates a pressure which tends to oppose localization of the electron. In an atom the pressure is opposed by the force attracting the electron back to the nucleus. Thus the electron will come to equilibrium when the attractive forces balances the effective pressure and, this way , the mean radius of the lowest quantum state is determined. This point of balance can be found from the condition that total energy must be minimum. Thus we have
W≅  (h2/2m(Δx)2) - (e2/Δx)
Differentiating both the sides w.r.t. Δx and making ∂W/∂(Δx) = 0 we get
Δx≅h2/me2
THis result is just the radius of first Bohr orbit although not exact but qualitative.. The limitation of the localizability of the electron is inherent in the wave-particle nature of matter. In order to have an electron in very small space , we must have very high fourier components in its wave function and therefore the possibility of very high moments. There is no way to force an electron to occpy a well defined position and still remain at rest.